1. Field of the Invention
This invention relates to the manufacture of synthetic hydrocarbon products from coal and similar carbonaceous solids and is particularly concerned with overcoming corrosion problems of the pressure reducing systems used in these processes.
2. Description of the Prior Art
Processes for the production of synthetic hydrocarbon products by liquefaction or gasification of coal and similar carbonaceous solids normally require direct contacting of the solid feed material with molecular hydrogen at elevated temperatures and pressure, with or without catalysts, to break down complex high molecular weight starting material into lower molecular weight hydrocarbon liquids and gases. There are numerous methods of coal liquefaction which achieve this result through different means. Over 170 such processes for the manufacture of synthetic hydrocarbon products from coal are described in "Oil from Coal" by Francis W. Richardson, Noyes Data Corporation (1975). Among these experimental processes being investigated, three liquefaction processes are in the more advanced stages of evaluation and show commercial promise: (1) the Solvent Refined Coal (SRC) process, (2) the Donor Solvent process and (3) the H-coal process. These processes are currently being scaled up from pilot plant to demonstration or semi-commercial plant size.
U.S. Pat. No. 3,640,816 to Bull and Schmid describes the Solvent Refined Coal (SRC) process as a multiple-stage non-catalytic hydrogenation process for producing light liquids from coal in which a slurry of pulverized coal, a solvent therefor and hydrogen are charged under pressure into a first reaction zone where the temperature is elevated and maintained until substantially all of the coal is dissolved. Gases and light liquids produced by partial hydrogenation of the reaction products are separated from the heavy bottoms and the latter are charged to another reaction zone under pressure where the charge along with an added quantity of hydrogen are heated to a higher temperature than present in the first zone so as to hydrocrack the constituents and produce additional quantities of gas and light liquids which are then separated from the heavy bottoms. The gases and light liquids from each stage are selectively segregated in a separation and distillation unit. Two or more reaction zones in series relationship may be employed with the charges being subjected to treatment conditions of successively increasing severity accomplished by successively higher temperatures, pressures or residence times or combinations of these parameters. The heavy bottoms from one or more of the stages may be recycled back to preceding stages if desired. A 50 ton per day SRC unit is currently operational.
U.S. Pat. No. 3,488,279 to Schulman describes the Donor Solvent Process in which coal is hydrogenated to produce liquid products in two stages. The first stage is an initial mild conversion by hydrogen donor extraction followed by a second stage of catalytic hydrogenation using a cobalt molybdate catalyst and added molecular hydrogen. By this sequence, conversion of oxygen to carbon dioxide rather than water is maximized, thus more efficiently using the hydrogen to form hydrocarbon products. The liquid products can be hydrocracked with a catalyst similar to that used in catalytic hydrogenation, and preferably the spent hydrocracking catalyst can be employed in the catalytic hydrogenation stage. A 250 ton per day plant using this process is being constructed.
U.S. Pat. No. 3,540,995 to Wolk and Johanson describes the H-coal process for converting coal to a light crude distillate by hydrogenation in an ebullated catalyst bed reactor. The process is directed to increasing the conversion of coal into hydrocarbons by recycling of slurry oil and controlling the composition, recycle rate and solids content of recycle liquid to the ebullated bed reactor. The largest U.S. Plant utilizing this process (600 ton per day) produces 72,000 gallons per day of naphtha which can be converted to 72,000 gallons of gasoline.
All of the aforementioned processes are plagued with severe corrosion problems due to the corrosive nature of the hot coal liquids and the resultant liquid and gaseous reaction products. This corrosion is aggravated by the high reaction temperatures and pressures employed. Particularly susceptible to this corrosion attack by hot coal liquids and gases are slurry pumps and pressure letdown valves. Extensive research into this problem is being carried out to develop new valve designs and valve materials. The importance in overcoming these problems is highlighted in a recent publication entitled "Inside D.O.E." page 9, McGraw-Hill, Nov. 9, 1979, in which it is noted that valves used in the Donor Solvent process which were made of conventional metals lasted only a few days and even valves made of such exotic material as tungsten carbide have to be replaced every 15 to 30 days. Not only is the replacement of the pressure letdown valves expensive per se but the resulting production down time escalates operating costs. Thus, there is an urgent need to solve problems of corrosion particularly in pressure letdown units before commercial coal liquefaction facilities are built.
U.S. Pat. No. 3,211,135 to Grimes et al relates to a pressure breakdown control system for a once-through high pressure steam generator. This system is said to minimize or eliminate rapid erosion, objectionable vibrations and disturbing noise conditions which are inherent in the start-up and shutdown sequences of this type of steam generating equipment. The Grimes et al system employs one or more pressure reducing tubes that define relatively long flow paths. Nothing in Grimes et al that suggests the use of this type of pressure breakdown means would overcome the significant corrosion problem unique in coal liquefaction or coal gasification environments. The steam present in Grimes et al is not highly corrosive as are the coal liquefaction and gasification products of the present invention. Corrosion is primarily a chemical phenomenon while erosion is primarily a physical phenomenon. In many corrosive environments metal surfaces are protected by the in situ formation of complex metal oxide layers or other protective coatings. This very thin coating, in effect, seals off the metal surface from corrosive attack. In corrosive environments which also entail rapidly moving fluid streams, there is often a deleterious effect to metal surfaces from the combination of chemical corrosion and physical erosion. The rapidly moving fluid stream may erode, by abrasion or scraping, the thin protective coating on the metal surface. The fresh surface is then quickly oxidized again to form a new protective coating. If this process is continuously and rapidly repeated, the metal surface will be worn away as a result of alternate formation and removal of the oxide layer. In the present invention, pressure reduction is effected in such a manner that corrosion is significantly reduced. This is accomplished by limiting the local velocity of the corrosive fluid so that the protective oxide layer is not continuously and aggressively damaged.